The present invention relates to an implant having varying modulus of elasticity. In the prior art, structures exist which either inherently or intentionally act to vary the modulus of elasticity along a desired length. However, Applicant is unaware of any such structure which includes all of the features of the present invention and acts to vary the modulus of elasticity of an elongated stem-like structure with the effectiveness of the present invention.
The following prior art is known to Applicant:
U.S. Pat. No. 3,906,550 to Rostoker, et al. discloses a prosthetic device having a porous fiber metal structure. As best seen in FIG. 1 thereof, a plurality of tubular fiber metal segments are disposed on a single elongated rod 24 and are provided to facilitate bony ingrowth along a large surface area. The present invention differs from the teachings cf Rostoker, et al. for reasons including the interpositioning of solid metallic discs between regions of fiber metal discs.
U.S. Pat. No. 4,051,559 to Pifferi discloses a prosthesis including a plurality of components assembled together to form the prosthesis. As seen in FIGS. 1 and 3 in particular, a stem 1 is attached to a head 5, 6, 7 through the use of an elongated bolt-like fastener 4. The present invention differs from the teachings of Pifferi for reasons including the use of regions of fiber metal discs separated by solid metallic discs.
U.S. Pat. No. 4,292,695 to Koeneman discloses a prosthesis stem including a stem portion formed in the shape of the medullary canal of a particular bone and including a plurality of individual sections of a resilient generally circular layer of elastomeric material 15 bonded in alternating fashion to a correspondingly shaped but rigid layer 17 made of a metallic alloy. The present invention differs from the teachings of Koeneman for reasons including the fact that the regions of the present invention composed of fiber metal may be varied as to their respective thicknesses between respective solid metallic discs to vary the modulus of elasticity of the stem portion thereof along its length.
U.S. Pat. No. 4,266,302 to Tornier discloses a femoral pin including a hilt member from which a plurality of funicular elements extend. The present invention differs from the teachings of Tornier for reasons including the fact that the present invention contemplates stacking of components of the prosthesis on top of one another rather than providing such components in a side-to-side configuration as is the case with Tornier.
U.S. Pat. No. 4,314,381 to Koeneman discloses a hip joint prosthesis having an elongated stem having mounted thereon radially outwardly protruding rings 35 and spacers 37, with the rings being made of an elastomeric material and with the spacers being made of a metallic alloy. The present invention differs from the teachings of Koeneman as contemplating a stacked relationship between rigid and less rigid components as compared to the radial spacing of the Koeneman components.
U.S. Pat. No. 4,743,263 to Petrtyl, et al. discloses a prosthesis which is similar to the teachings of Tornier, discussed above, as including a plurality of strands of material emanating from a single hilt. The present invention differs from the teachings of Petrytyl, et al. for the same reasons set forth above concerning Tornier.
U.S. Pat. No. 4,808,186 to Smith discloses an implant having an elongated stem with a longitudinal channel therein. The thickness of the stem laterally of the channel is variable between the proximal and distal ends to achieve stem flexibility which substantially correlates to the flexibility of the bone. The present invention differs from the teachings of Smith for reasons including the interpositioning of solid metallic discs between regions of fiber metal discs.
European Patent Application No. 0 220 803 to Brooks, et al. discloses a prosthesis including a cushioning body 15 interposed between the head 13 thereof and the elongated stem 11 thereof. The present invention differs from the teachings of Brooks, et al. as contemplating a variability in the modulus of elasticity of an elongated stem component throughout its length.
French Patent No. 2 425 237 to Breard discloses a prosthesis including, in a manner similar to the teachings of Tornier and Petrytyl, et al. discussed above, a plurality of strands of material designated by the reference numeral 9 in FIG. 6, for example, emanating from a single location. Again, the present invention differs from the teachings of Breard for the same reasons discussed hereinabove with regard to Tornier.
It is a known fact that live bone continuously changes in response to the demands and forces placed on it. Thus, a well designed prosthesis should include consideration of factors allowing duplication of the properties of the bone which is adjacent the prosthesis.
A prosthesis should transmit load evenly and over a large area while at the same time distributing continuous stress to the adjacent bone sufficient to encourage healthy bone formation throughout the interface therebetween (for a prosthesis of the type that is adapted for bony ingrowth, as with a porous material such as with a porous fiber metal structure). Thus, it is desired to provide a prosthesis which includes variations in the modulus of elasticity in different regions thereof, excellent fatigue properties, and maximum exposure of the bony ingrowth regions thereof to best facilitate bony ingrowth.
The present invention seeks to provide a prosthesis which:
(1) is less rigid than many of the presently manufactured prostheses;
(2) incorporates load transmission patterns which are designed to conform to normal bone characteristics;
(3) optimizes the area of bony ingrowth to enhance securement of the prosthesis to the bone, and thus lessen the possibility of prosthesis loosening and provide more even stress distribution.